This next step requires an admin-level account. You, or your admin, should create a group for any guides that will have their own banners. The Group Look & Feel settings should be set to Do not display a group banner.

Head to Modules > Add-on Module and then click View my image library. Upload the banner image for your guide.

Before you close that window, right-click on the image you've just uploaded. In Firefox, select Copy Image Location. In Chrome, pick Copy Image URL.

Mosey on over to your guide! Make sure your guide is assigned to the group you created in step #2. Click Guide Settings > Change Guide Information, and choose your group from the drop-down menu.